In general, a so-called “wet-process phosphoric acid” actually refers to wet-process phosphoric acid produced by a sulfuric acid process, that is, phosphoric acid is produced by decomposing a phosphate rock with sulfuric acid. In China, 80% or more of phosphoric acid is produced by a dihydrate process for wet-process phosphoric acid. The dihydrate process is characterized in that the process is simple, the technique is mature and the adaptability to rock types is high, which is especially suitable for low and medium grade rocks and dominates in the wet-process phosphoric acid production. However, there are still some insurmountable deficiencies in the dihydrate process. The application of by-product phosphogypsum is subjected to many limitations due to its high phosphorus content, of which only a small part is used to produce low-grade and low-quality gypsum building materials and cement retarders. Most of phosphogypsum is discarded or stockpiled, causing serious pollution and waste. Therefore, it is necessary to further improve the conventional wet-process phosphoric acid production so as to reduce the phosphorous content in phosphogypsum, thereby converting it into a product with high use value.
Chinese Patent (Publication No. CN103626143A) discloses a method for producing phosphoric acid and by-producing white gypsum by a wet process. Firstly, phosphate rock powder (pulp) reacts with phosphoric acid at 45 to 70° C. under stirring to generate a mixed pulp containing solid impurities. The mixed pulp is subjected to continuous or discontinuous precipitation and separation to give a mixed solution of phosphoric acid and calcium phosphate, as well as a thick pulp containing solid impurities. Under condition of stirring, sulfuric acid is added to the mixed solution of phosphoric acid and calcium phosphate and reaction undergoes. The resulting mixture is precipitated, layered and separated into phosphoric acid and white gypsum. By this method, phosphoric acid and white gypsum are obtained without the generation of phosphogypsum, eliminating the pollution from the piled phosphogypsum to air, soil and underground water. In addition, the by-product dihydrate white gypsum has high purity and high whiteness value. However, there are still acid-insoluble residues generated by this method, which is difficult to deal with. In addition, the dihydrate white gypsum needs to be subjected to dehydrate process or crystal modification process to obtain beta-gypsum or alpha-gypsum products with high added value.
Chinese patent (Publication No. CN102001636A) discloses a method for preparing phosphoric acid with a wide-range concentration and clean gypsum through wet-process from low or medium grade phosphate rock. The method provides a new method for preparing wet-process phosphoric acid-hemi-dihydrate method, of which the by-product is high quality construction hemihydrate gypsum or functional dihydrate gypsum, making full use of low or medium grade phosphate rock. However, the method still discharges solid residues and dihydrate gypsum at a similar amount of the clean gypsum, which is hard to be used.
Chinese Patent (Publication No. CN1421385) discloses a method for preparing hemihydrate-dihydrate phosphoric acid. In the method, the precipitation ratio of calcium in reaction tank is controlled and sulfuric acid is added at two steps. One part of sulfuric acid is added to acid-mixing tank, mixed with diluted phosphoric acid and then added to the second reaction tank; the other part of sulfuric acid is added to diluted phosphoric acid tank. Alpha-hemihydrate gypsum is prepared firstly and then transformed to dihydrate gypsum. By-product of the method is still dihydrate gypsum, which is hard to be used directly. Chinese Patent (Publication No. CN103086335A) discloses a dihydrate-hemihydrate method for producing wet-process phosphoric acid and coproducing by-product alpha-hemihydrate gypsum. ω(P2O5) concentration of the wet-process phosphoric acid produced by dihydrate-process is from 35% to 39%, and ω(P2O5) concentration of the wet-process phosphoric acid produced by hemihydrate-process is from 10% to 15% used as an acid supplemental production of dihydrate process. The by-product, hemihydrate phosphogypsum, contains 5% to 7% crystal water, in which the mass percentage of free P2O5 is less than 0.4%, and the crystal form of which is alpha-hemihydrate phosphogypsum. In the method, the condition for dihydrate-hemihydrate crystal transformation is not strictly controlled, and no crystal transformation agent is used to control the aspect ratio of alpha-hemihydrate gypsum. Although alpha-hemihydrate gypsum product is produced, the strength of alpha-hemihydrate gypsum product is low, so the use of the product is limited. In addition, phosphorus content of the product is still relative high.
In summary, the problems in the prior art are: 1) wastes such as acid insoluble substances still need to be discharged; 2) the coproduced gypsum is a dihydrate, which needs subsequent process to become products with a high added value, such as alpha-hemihydrate gypsum, beta-hemihydrate gypsum, etc.; 3) the resulting alpha-hemihydrate gypsum product has low strength and high phosphorus content, limiting its high-end applications.